check letters (or numbers) inside of a string

Hello

Here's the problem. I have these uons read from attributes in a drawing. The
value of the attribute should be something like 2345434, the amount of
numbers varies. The format of uons is string. Anyway my code has worked
perfectly... until now. I just noticed that if the value of uons is for
example VJH.80 an error is generated. This happens because the type of
uon_appl in database is number. So if I pass letters to sql statement it
causes an error.

How can I check the uons so that I go through every letter individually and
if it is not number I wont perform the sql sentence. Or if you have some
other solution for this I'd like to see that.

The first problem is that I can't convert uons to integer cause i have to
use strcat (or is there some other way I could do that same thing?)

The other one is that I don't want to use "like" statement instead of "="
(which would make the program work) because the program gets really slow
with "like" statement.

Here are the two lines that are causing problems:

(setq uons (cdr (assoc 1 ed2)))
(setq finds (caar (sqlexe (strcat "SELECT uon_appl FROM elina_application
WHERE uon_appl = '"uons"'"))))